Optical sources which can generate radiation over a wide wavelength range currently have many applications in scientific research, engineering and medicine. Often the application requires the coherence properties associated with laser radiation and so various laser materials have been developed which exhibit a broad gain bandwidth when suitably pumped. A good example is titanium doped sapphire, which is characterized by a gain bandwidth covering the range 650-1100 nm. The coherence properties of lasers based on broadband material systems may be further enhanced in a number of ways. The technique of modelocking permits the utilization of much of the available laser bandwidth to obtain a repetitive train of short (or ultra short) pulses. Alternatively, the laser cavity may contain frequency selective elements to ensure that the laser only emits radiation with a relatively narrow spectrum centered around a particular wavelength. By adjusting a frequency selective element this wavelength may be tuned across the available gain bandwidth.
However, as many laser sources only operate over narrower, well-defined wavelength ranges, nonlinear optical processes have been employed to generate other wavelengths using the output from available laser sources. A wide range of nonlinear optical processes are known, with the common factor being a nonlinear dependence of the electric polarization that is induced in the nonlinear material on the electric field (or intensity) of an optical input, resulting in the generated optical output. The order of nonlinearity relates to the specific integer power of the electric field on which the induced polarization depends. Second order effects include second harmonic generation, which is commonly used to “frequency double” the output from a laser source, and also two-wave mixing. Third order effects are more numerous and include third harmonic generation, four-wave mixing, self-phase modulation, self-focussing and Raman scattering.
As indicated, non-linear optical devices have the advantage that they can be “bolted on” to the output of existing laser sources in order to extend the available wavelength range or simply to generate nonlinearly another well-defined wavelength from the laser radiation. The strength of the nonlinear effect is generally determined by the relevant non-linear coefficient of the material and the peak intensity of the input (pump) beam inside the material. However, other factors such as interaction length and accurate phase-matching can be very important in maximizing the efficiency of conversion in the nonlinear interaction.
Nonlinear processes and materials capable of generating radiation over a wide wavelength range from a relatively narrow band optical input are of particular interest. Optical continuum generation (CG) is an example whereby a cascade of (generally) third order processes enables the generation of a coherent optical signal with a continuous, or near continuous, spectrum over a very broad bandwidth. The continuum generated can be used in its entirety or optically filtered or sliced as the application requires. Optical parametric processes are another example where one or more optical signals that are tunable over a wide bandwidth are generated from an input pump at a fixed wavelength. In particular, the optical parametric amplifier (OPA) provides parametric amplification at two tunable wavelengths (the signal and idler), whereas the optical parametric oscillator (OPO) employs (tunable) optical feedback at one, or both, of these wavelengths to achieve self-oscillation. A variety of techniques have been investigated for enhancing the bandwidth that can be accessed by the nonlinear processes described above.
Optical CG has typically been performed in bulk materials, both liquid and gas, due to the simplicity of implementation and the relatively small sample size required. However, due the low nonlinear coefficient associated with many materials, the characteristic threshold intensity is high and so a high peak intensity laser source is required. This usually takes the form of a modelocked laser system generating very short (or ultrashort) pulses which are then amplified and the radiation focussed tightly onto the target material. Consequently, there is a high attendant risk of surface or bulk damage to the sample unless it is a material exhibiting a high damage threshold such as sapphire, which also exhibits good stability of CG.
One approach to reducing the threshold pulse energy required has been the utilization of optical fibers for CG. Despite a relatively low nonlinear coefficient for the fiber material, the lateral optical confinement ensures that an adequate optical intensity can be maintained throughout a long interaction length of fiber for efficient CG. Nevertheless, the associated pulse energy damage threshold is also reduced and so end facet damage may occur, requiring the cleaving of a new facet or provision of a new fiber entirely. In addition, the stability of CG in optical fibers is typically low, the overall size may limit the compactness of the source and the optical mode properties are not easily compatible with the planar waveguide devices used in photonic integrated circuits.
Another issue associated with CG is the characteristic optical dispersion (variation of refractive index with wavelength) of the device in which the continuum is generated. It is known that the threshold for CG is lowered when the pump is at a wavelength where the dispersion of the device is near zero. Furthermore, due to the proximity of the anomalous dispersion region, more nonlinear processes may be accessed and bandwidth may more easily be generated beyond the zero dispersion wavelength, extending the spectrum further into the infrared. In the case of bulk materials, the characteristic dispersion is simply the material dispersion, which usually lies within the normal dispersion region at the optical wavelengths of interest. But for waveguides, the situation is more complex, with the total dispersion also depending on waveguide and modal dispersion. This provides scope for controlling the total dispersion via the waveguide design parameters. To this end, zero dispersion fibers and tapered fibers have been manufactured.
A further development in the control of fiber dispersion has been the fabrication of the so-called microstructured fiber (MF) or photonic crystal fiber (PCF). These fibers consist of a solid silica core surrounded by an array of air holes running along the fiber, which provides a wavelength-dependent effective index for the cladding and can allow single-mode guidance throughout the visible and near infra-red. By suitable choice of arrangement and size of holes, the dispersion properties of the fiber can be tailored, as can the effective area of the propagating mode. Such dispersion engineered fibers have been used to enhance the continuum bandwidth that can be generated from a short pulse pump, resulting in so-called supercontinuum generation. Here, bandwidth in excess of 800 nm has been generated as a result of a cascade of processes, including self-phase modulation (SPM), four-wave mixing (FWM), Raman scattering (RS), soliton formation and decay, soliton self-frequency shifting (SSFS) and self-steepening (SS). However, despite the improvement in CG bandwidth, the microstructured fibers still suffer from the drawbacks associated with more conventional fibers, as outlined above. In addition, there are many materials that can not be fabricated in bulk or fiber form and are therefore unavailable for CG in these configurations.
Another waveguide based approach for enhancing a nonlinear phenomenon has been the employment of a planar chalcogenide glass (ChG) waveguide. Here, the intention was to enhance the level of nonlinear phase shift that could be obtained via SPM for a given pump pulse energy, a key application being optical switching for optical communication systems. A thin film of high refractive index GeSe-based glass material formed the core of a planar waveguide that was subjected to pump pulses from an amplified mode-locked fiber ring laser. A maximum peak phase shift of 1.6π was recorded for an input pulse energy of 461 pJ. However, although the process was accompanied by nonlinear spectral broadening, which resulted in an optical output having a bandwidth broader than that of the input pulse, the degree of spectral broadening was not sufficient to generate an optical continuum.
Planar waveguides have also been employed for enhanced performance in parametric devices, such as the optical parametric oscillator (OPO) and optical parametric amplifier (OPA). Typical devices comprise a layer of periodically poled material such as Lithium Niobate (Li2O3), also known as PPLN. Although improved performance is obtained in terms of threshold power and conversion efficiency, the available tuning range is still limited.